Correct Answer - Option 2 : 7 kmph
Given: The person can row at a speed of 21 kmph in still water
Formula used: u/v = (x + y)/(x - y) where u is speed of rowing downstream, v is speed of rowing upstream, x is speed in still water and y is speed of stream.
Calculation: He takes twice long to row a distance upstream as to row same distance downstream, therefore, u = 2 × v,
⇒ u/v = 2/1 = (x + y)/(x - y)
Using Componendo and Dividendo rule, we get
[(x + y) + (x - y)] / [(x + y) - (x - y)] = [2 + 1] / [2 - 1]
⇒ 2x / 2y = 3/1
⇒ x / y = 3/1
⇒ y = x/3 = 21/3 = 7 kmph.
∴ The speed of the stream is 7 kmph.
